Some hybrid vehicles may include a motor hybrid transmission (MHT). Therein, a driveline disconnect clutch can mechanically and selectively isolate an engine from the transmission and vehicle wheels so that the transmission and wheels can operate independently from the engine. The driveline disconnect clutch allows torque to be provided to the driveline to propel the vehicle even if the engine has stopped rotating. In addition, the system may include an electric motor located between the flex-plate and the torque converter and can be used to supplement torque output as well as absorb and store power during vehicle deceleration.
Typically, engine powertrain control systems perform torque control during transmission shifting to help match engine speed for the next gear being commanded by the engine. One example approach for torque control is shown by Badillo et al. in U.S. Pat. No. 6,770,009. Therein, spark retard is used during a vehicle launch from rest to improve torque delivery between a clutch plate on the engine side and a clutch plate on the transmission side. The fast actuation of the spark retard reduces unstable torque control during the vehicle launch.
However the inventors herein have recognized that torque control in hybrid vehicles having motor hybrid transmissions may be complicated. This is due to the increased inertia weight of the rotating assembly of the engine and the armature of the hybrid motor combining to make significant inertial mass that may not be decelerated fast enough to match the transmission shift speed. As a result, there may be conditions where utilizing spark retard does not provide the desired smooth transition, resulting in recurring vehicle bucking and NVH issues. Specifically, during a transmission shift, closing the driveline disconnect clutch may induce a noticeable torque disturbance into the driveline if the driveline disconnect clutch transfers more torque than is desired due to a large speed difference between either sides of the clutch.
In one example, the above issue may be at least partly addressed by a method for a hybrid vehicle, comprising, during a transmission gear shift, reducing engine speed by operating one or more cylinders with spark timing advanced of MBT, and when engine speed is lower than a threshold, operating the one or more cylinders with spark timing retarded from MBT. In this way, enough negative torque can be produced to slow the engine for a smooth transmission shift.
As an example, during a transmission shift in a hybrid electric vehicle configured with a motor hybrid transmission, spark timing may be advanced from MBT in one or more engine cylinders to generate sufficient negative torque to slow engine speed. The amount of spark advance used may be selected so that a peak pressure of the cylinder occurs well before TDC so as to reduce cylinder knocking. Based on the amount of torque reduction needed, one or more cylinders (e.g., all cylinders) of the engine may have spark timing advanced. For example, over the given transmission shift, some cylinders may have spark timing advanced of MBT while other remaining cylinders have spark timing retarded of MBT, or at MBT. The use of spark advance allows the engine speed to be reduced to a threshold speed faster. Optionally, in engines configured with direct fuel injection, a stratified charge may be used along with the spark advance to improve combustion. The stratified charge may include, for example, injecting fuel near the time of spark. Once the engine speed is at or below the threshold speed, the use of spark advance during the transmission shift may be stopped. Instead, spark retard may be used in the one or more engine cylinders to reduce the propensity for knock during the shift. In addition, use of spark retard may be used after the transmission shift.
In this way, by advancing spark timing from MBT during a transmission shift in a hybrid vehicle, more negative torque can be produced earlier in an engine cycle. In particular, while spark retard from MBT reduces torque, spark advance well in advance of MBT can produce negative torque, as the piston has to do work against the pressure created by combusting very early. As such, this approach may generate more negative torque than may be produced using fuel shut-off to all engine cylinders, or a closed throttle deceleration. By increasing the amount of negative torque produced, engine speed can be rapidly slowed to match the engine speed required during the shift. Overall, a smoother transmission shift is enabled.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.